Agricultural combines typically have two driven wheels to propel them through the field. The use of two driven wheels limits their performance. With only two driven wheels, combines are prone to slip. To avoid this problem, prior art combines were designed that use a four wheel drive arrangement in which hydraulic fluid from a hydraulic pump is directed to a single motor coupled to two front wheels to drive the front wheels and fluid is also directed to two motors, each motor driving a single rear wheel. This arrangement required the use of a flow splitter circuit that divides the output of the flow between the front wheels' drive motor and the rear wheels' drive motors. The flow splitting circuit was designed to insure that some flow was forced though the front drive motor as well as through the two rear drive motors. In this fashion, if a front wheel slipped, all the hydraulic fluid would not be directed through the front drive motor and bypass the rear drive motors. Instead, and due to the arrangement of the splitter, at least a portion of the flow would be forced through the rear drive motors. By dividing the flow and insuring that at least a portion of the flow would go through both the front motor and the rear motors even when there was slippage of a wheel, slippage of a single wheel would not cause the combine to immediately stop moving, since at least some fluid would continue to drive other motors coupled to wheels that have good ground contact.
This system, however, has drawbacks. It requires the use of a hydro-mechanical flow splitter. Furthermore, it still permitted both front wheels to be disabled as soon as one of them started slipping and for both rear wheels to be disabled as soon as one of them started slipping.
It is an object of this invention to provide a combine with an improved anti-slip drive system.
It is an object of this invention to provide a drive system that can isolate any individual drive wheel that begins slipping.
It is an object of this invention to prevent the slippage of a wheel by controlling the specific displacement of the motor that drives the slipping wheel.
It is an object of this invention to prevent the slippage of a wheel by reducing the torque provided by the motor driving that wheel.
These and other objects of the invention will become clear upon reading the following summary and detailed description of a preferred embodiment of the invention.